Sintering is a process in which adjacent surfaces of metal powder particles are bonded by heating. Liquid phase sintering is a special form of sintering during which solid powder particles coexist with a liquid phase. Densification and homogenization of the mixture occur as the metals diffuse into one another and form new alloy and/or intermetallic species.
In transient liquid phase sintering (TLPS) of powders, the liquid phase only exists for a short period of time as a result of the homogenization of the metals to form a mixture of solid alloy and/or intermetallic species. The liquid phase has a very high solubility in the surrounding solid phase, thus diffusing rapidly into the solid, and eventually solidifying. Diffusional homogenization creates the final composition without the need to heat the mixture above its equilibrium melting point.
In TLPS compositions comprising powder metallurgy, a relatively low melting point (LMP) alloy and a relatively high melting point (HMP) metal are mixed in particulate form. At least one element within the alloy is either highly soluble in, or is reactive with, the receptive HMP metal. As the temperature is raised to the melting point of the LMP alloy, the alloy particles become molten. This transition can be observed as an endothermic event in differential scanning calorimetry (DSC). The reactive element(s) within the relatively low melting alloy then react with the receptive high melting point metal to form new alloy compositions and/or intermetallics. The formation of intermetallic species may be observed as an exothermic event using DSC. Thus, the typical TLPS DSC “signature” is an endotherm followed by an exotherm. The diffusion and reaction of the reactive element(s) from the low melting alloy and the receptive high melting metal continues until one of the reactants is fully depleted, there is no longer a molten phase at the process temperature, or the reaction is quenched by cooling. After cooling, subsequent temperature excursions, even beyond the original LMP alloy melt temperature, do not reproduce the original melt signature of the mixture. This is the “signature” of a typical low temperature transient liquid phase sintered metal mixture.
TLPS technology is used to produce organo-metallic conductive compositions that include HMP metal powder(s), LMP metal alloy powder(s), and a permanent adhesive-flux polymer system. TLPS compositions are employed, for example, to form conductive paths on printed circuits by creating a patterned deposition of the TLPS composition, and then simultaneously sintering the metallic components and curing the adhesive/polymer components in the composition by heating at relatively low temperature. During heating, the adhesive-flux polymer fluxes the metal powders, allowing TLPS to occur. After heating, the adhesive-flux chemically binds the resultant metal oxides, rendering them harmless. For this reason, these compositions provide good electrical conductivity with little opportunity for conductivity deterioration due to oxidation, corrosion or thermal expansion and contraction.
The microstructure of processed TLPS compositions appears as a network of particles of HMP metal, each bearing one or more “shells” of the newly formed alloy/intermetallic compositions, which are in turn interconnected by the non-reactive portion of the original LMP alloy. Open areas of the metallic network structure are generally filled with the cured polymeric binder. Reaction between the HMP metal and the reactive element(s) of the LMP alloy may result in either partial or complete incorporation of the HMP metal particles into the newly formed alloy and/or intermetallic species. The number and nature of the new alloy and/or intermetallic species that form is dependent on the selection of metallic constituents in the TLPS composition, their relative proportions, the particle size distribution and the process temperature. The composition of the residual components of the original LMP alloy is likewise dependent on these factors.
TLPS compositions are suitable replacements for conventional electrically and/or thermally conductive materials in a diverse assortment of applications, including assembly of electronic components, deposition of in-plane circuit traces, interconnection of circuit traces on different planes, assembly of unpackaged integrated circuit die onto packaging elements, and the like. For each of these applications, there is a specific set of application-specific attributes for which TLPS compositions confer an advantage over conventional materials. Attributes include, but are not limited to, ease of deposition, reduction in manufacturing time or complexity, increased circuit density in the resultant article, and production of environmentally stable interfaces that have high electrical and/or thermal conductivity.
However, in each of these applications the TLPS compositions directly contact, and in some instances are surrounded by, materials with different mechanical characteristics and coefficients of thermal expansion (CTE). Typically, it is desirable for the TLPS composition to sinter with adjacent metallic circuit and/or electronic component elements in the electronic device as well as within the composition itself. Although reaction between the TLPS composition and adjacent TLPS-receptive elements confers protection of the interface from some environmental interference, it also mechanically couples the composition to the adjacent elements, which may be either beneficial or detrimental depending on the application. Further, in several applications, the TLPS material is used under circumstances in which a polymeric component (e.g. circuit substrates in printed circuit boards) imposes an upper limit to the process temperature and duration, and thus restricts the selection of LMP alloy(s) to those that are compatible with the polymer. In addition, such polymer constituents may have substantially different CTE than the TLPS composition, and other surrounding elements, and may be exposed to large changes in CTE during process steps in which the glass transition temperature of such polymeric constituents is exceeded. These factors must be taken into account in order to provide a durable, reliable end product.
The demands on the metallic constituents in TLPS applications are high. Some of the critical features of the metallurgy include:                The melting point of the LMP alloy must be sufficiently low that processing of the composition does not damage the surrounding materials.        The LMP alloy and HMP metal must form species that are good electrical and/or thermal conductors.        The reaction products of the LMP alloy and HMP metal must be stable over the likely thermal exposure range.        The metallic network formed by the TLPS process must be resistant to the deleterious effects of mechanical stress.        The LMP alloy must be compatible and reactive with the metal finishes of the circuit elements to be connected.        The primary constituent LMP and HMP metals should be readily available at a reasonable cost.        The constituents should not be restricted due to environmental or toxicity concerns.        
The optimum choice for the HMP metal is typically copper, although some alternatives may be useful in specific applications (e.g. resistors). Copper, which has a melting point in excess of 1000° C., is relatively inexpensive, plentiful, readily available in a variety of powder forms, compatible with the metallurgy typically used for circuit elements, ductile, and is an excellent electrical and thermal conductor. Although more expensive, silver, indium, gold, and germanium are also suitable HMP metals for use in TLPS composition. Aluminum is also contemplated.
The selection of a suitable LMP alloy material is more challenging. The first challenge is process temperature. The process temperature for TLPS compositions in the electronics industry must be low enough that the other materials used in the production of an electronic article are not damaged. For electronic applications that include polymeric components, a ceiling of 250° C. is typically the maximum permissible process temperature and therefore, the low melting temperature alloy is generally limited to alloys of tin, bismuth, lead, gallium, indium, and zinc. Alloys of lead are excluded due to toxicity. Alloys of gallium and indium are prohibitively expensive and not readily available. Alloys of zinc are incompatible with many common circuit finishes. However, alloys of tin with bismuth have reduced melting temperatures as compared to tin alone, and may therefore present the best combination of attributes, although the addition of other elements may be desirable to achieve specific characteristics.
Alloys of bismuth and tin meet many of the objectives outlined above; however, they also present some deficiencies. Bismuth-tin alloys are readily available at reasonable cost in particulate form. The tin in the bismuth-tin alloys, and the intermetallics formed when tin reacts with copper, are all very good electrical and thermal conductors. The residual, elemental bismuth and the copper-tin intermetallics formed during a TLPS reaction all present melting temperatures outside the range of subsequent thermal processing and testing typically performed. Tin and bismuth are not considered toxic and are compatible with all commonly used metal circuit finishes. Unfortunately, both bismuth and copper-tin intermetallics are brittle and therefore susceptible to damage when exposed to mechanical stress. Further, elemental bismuth is a poor electrical conductor and a very poor thermal conductor.
As alloys of bismuth and tin used in conjunction with copper as the HMP metal offer most of the desired characteristics, it would be advantageous to provide a means to employ this metallurgy in a manner that minimizes its deficiencies. Bismuth is at least partially responsible for the primary deficiencies of low electrical and thermal conductivity, and brittleness. Therefore, it would be desirable to minimize the proportion of bismuth in TLPS compositions used in electronic applications.